Platelets function both mechanically and biochemically in the process of hemostasis. When injury to a blood vessel occurs, platelets aggregate, forming a primary hemostatic plug, which helps to stop the flow of blood.

Platelets also release certain substances, among them serotonin and platelet factor 3. Serotonin causes the blood vessels in the area to constrict, thereby further stopping the flow of blood. Platelet factor 3 catalyzes the coagulation reaction, which results in the formation of a fibrin clot. Platelets also maintain the integrity (leak-free) state of blood vessels.